Abstract

We adopted simultaneously three effects of milling, carbon (C) doping and solid-state reaction to improve critical current properties of MgB2 superconducting wire. The influences of heat-treatment temperature, including a conventional solid-liquid reaction of 650-1000 degrees C and a solid-state reaction of 600 degrees C below the melting point (650 degrees C) of Mg powder, were examined on the transport superconducting properties of in situ powder-in-tube (PIT) processed MgB2/Fe wires using ball-milled and glycerin-treated boron (B) powder. The aims of the mechanical milling and liquid glycerin treatment of the B powder were to reduce the grain size of the MgB2 and achieve homogeneous C incorporation into the MgB2, respectively. The superconducting properties of MgB2 wires heat-treated in the range of 650-1000 degrees C were investigated, and it was also investigated as to whether the C incorporation occurred even in a low-temperature solid-state process of 600 degrees C, thus resulting in an improvement of the superconducting properties by obtaining both high grain boundary density and C substitution effects. The MgB2 phase formation, actual C substitution amount, full width at half maximum (FWHM), critical temperature (T-c), magnetic field dependence of transport critical current density (J(c)) and temperature dependence of the upper critical field (H-c2) were evaluated for glycerin-doped MgB2/Fe wires fabricated at different heat-treatment temperatures. The glycerin-doped MgB2 wire using milled B powder heat-treated at a solid-state of 600 degrees C showed the highest transport J(c) values at 4.2 K over the entire applied field regime. It was revealed that the grain boundary density was higher and that the C substitution also occurred by a low temperature heat-treatment process, which led to a higher J(c). In addition, a solid-solid diffusion reaction with the pre-treated B powder resulted in poor crystallinity, which enhanced H-c2 and improved J(c). (C) 2015 Elsevier B.V. All rights reserved.